Complete chemical system for a kraft mill



United States Patent 3,366,534 COMPLETE CHEMICAL SYSTEM FOR A KRAFT MILL William Howard Rapson, Toronto, Ontario, Canada, as-

signor to Hooker Chemical Corporation, Niagara Falls,

.Y., a corporation of New York Filed Aug. 14, 1964, Ser. No. 389,758 Claims. (Cl. 162-30) This invention relates to the manufacture of bleached pulp and to processes useful in connection with the manufacture of bleached pulp.

In some pulping processes, such as the kraft process, the Alkafide process that is, using 100 percent sulfidity cooking liquor, etc., cellulosic material most frequently Wood chips, but also other fibrous material such as grasses, straw and bagasse, for example, is heated in the presence of a solution containing sodium sulphide or sodium sulphide and sodium hydroxide through a timetemperature cycle, during which a large part of the lignin, hemicelluloses, and extractives are dissolved, and the major portion of the cellulose in the cell Walls remains in the form of fibres suspended in the solution. The fibres are separated and washed, the solution, called black liquor is evaporated, burned in a furnace, dissolved in water, clarified and causticized to make the so-called white liquor, a solution containing sodium hydroxide,

sodium sulphide, and small amounts of sodium carbonate and sodium sulphate, which is used to make up the cooking liquor for additional quantities of cellulosic material. The losses in this cyclic system are made up by adding sodium sulphate, usually to the evaporated black liquor entering the furnace. The washed pulp then may be bleached, usually using combinations of chlorine, sodium hydroxide, sodium hypochlorite, and chlorine dioxide in various bleaching stages. The bleaching chemicals are not normally recovered and processed for re-use, but are discharged from the system to waste.

For this commonly used process, pulp mills normally purchase sodium sulphate for making up the losses in the pulping chemical recovery system; elemental chlorine for bleaching; sodium hydroxide for caustic extraction and for manufacture of sodium hypochlorite; sodium chlorate; sulphuric acid; and one of sulphur dioxide, methanol or sodium chloride for reducing sodium chlorate to chlorine dioxide for use in bleaching.

One inventive concept disclosed herein relates to a process for providing all the necessary chemicals for cooking and bleaching pulp to high brightness, with good colour stability and good strength, from only sodium chlorate, sodium chloride, and sulphuric acid, and, Where necessary, some lime, quick limes, or limestone to make up for the losses in the normally used cyclic causticization process, eliminating the need for purchase or separate manufacture of elemental chlorine, sodium hydroxide, and sodium sulphate by the pulp mill.

In accordance with an extension of the foregoing concept it also is possible to manufacture sodium chlorate at the pulp mill by electrolysis of brine, and/ or manufacture sulphuric acid from elemental sulphur, in which case, in addition to electricity, air and water, only sodium chloride, sulphur and limestone, all of which are relatively cheap, naturally occurring chemicals, are required by the pulp mill.

Another inventive concept disclosed herein involves not only using white liquor in place of sodium hydroxide in the caustic extraction stages of bleaching, but also the recovery of the chemicals and dissolved organic matter remaining in the solution accompanying the pulp for example, by concentration, by recycling, separation from the pulp and reintroduction into the chemical recovery system.

This inventive concept may be integrated with the hereinbefore mentioned processes to provide another cyclic process which diminishes still further the amounts of chemicals which must be introduced into the pulping and bleaching process. Thus, recovery and re-use of the white liquor in this manner and after using it for caustic extraction decreases the amount of sodium sulphate required for producing white liquor. This in turn makes it possible to operate the chlorine dioxide generator to produce more chlorine dioxide, less chlorine and less sodium sulphate. This in turn diminishes the amount of sodium chloride and sulphuric acid which must be added to the chlorine dioxide generator, sharply reducing the total amount of chemicals per ton of pulp which must be purchased or manufactured by the mill and sharply reducing the total cost of the chemicals required for producing bleached pulp.

An additional benefit derived from this integrated process where recovery of the chemicals is carried out by the specific steps hereinbefore mentioned is the saving of the heat which normally must be added to the pulp at about 12 percent consistency in the usual caustic extraction stage. A further benefit of the general integrated process is recovery of the heat of combustion of the organic matter dissolved from the pulp during caustic extraction and then recovered. Another feature of the general integrated process is the elimination of stream pollution by virtue of the fact that the liquid efliuent from the caustic extraction stages of bleaching is no longer rejected as waste, as is the normal current practice.

One of the major problems in any complex sytsem which provides three different chemicals, in this case chlorine dioxide, chlorine and salt cake in accordance wi the following reactions 1 and 2:

which chemicals may be required individually in different amounts, is the matter of balancing the amounts of chemicals produced to fit the needs of the system utilizing the chemicals.

The basic process disclosed herein is capable of providing each of the chemicals required by a bleached kraft pulp mill in difierent relative but related amounts over a very wide range by increasing the amount of sodium chloride and sulphuric acid relative to the amount of sodium chlorate fed to the reactor, the former chemicals being fed in approximately the ratio of two moles of sodium chloride to one mole of sulphuric acid. Such additional salt and sulphuric acid will produce chlorine and salt cake according to Reaction 2 by diverting part of the sodium chlorate from Reaction 1.

Thus, it is possible by means of this basic process to provide all the oxidizing power required for bleaching pulp in all stages of bleaching by means of sodium chlorate, which produces chlorine and chlorine dioxide according to Reactions 1 and 2 in a ratio which is determined by the ratio of sodium chloride to sodium chlorate fed to the reactor. When additional salt cake is required, it generally can be made simply by increasing the rates of flow of sodium chloride and sulphuric acid into the reactor, thereby increasing the ratio of chlorine to chlorine dioxide produced in the reactor. This variable ratio of chlorine to chlorine dioxide can be accepted in the bleaching system, since chlorine dioxide is used preferentially in the later stages of bleaching, while a wide range of mixtures of chlorine dioxide and chlorine may be used in the first stage of bleaching and are superior to either chemical alone, as disclosed in my pending U.S. Patent Application Ser. No. 378,417.

Thus, it is possible to balance over a very wide range the requirement for total available chlorine for bleachbasic process, allows bleached ulp to be manufactured starting only with cellulosic material and the three chemical materials, sodium chloride, sulphuric acid and limestone (or lime), two of which occur naturally.

ing and sodium sulphate requirement for make-up losses 5 As another extension of the basic process, the sulphuric in the kraft pulping chemical recovery system, and for acid used in the generator may be manufactured by the the provision of white liquor containing sodium hypulp mill from sulphur, air and water. This step, when droxide and sodium sulphide for use in the caustic exintegrated with the basic process, allows bleached pulp traction stages of bleaching as herein described. to be manufactured starting only with cellulos1c materlal The balance can be obtained by introducing the and the four chemical materials, sulphur, sod um chlorate, amounts of chemicals shown in Table 1 into the chlorine, sodium chloride and limestone or lime, three of which chlorine dioxide and sodium sulphate generator. This occur naturally. table shows the amounts of all chemicals introduced into As a still further extension of the basic process, the and produced in the generator when 100 pounds of electrolysis of sodium chloride in an electrolytic cell by sodium chlorate react in varying proportions according the pulp mill and the manufacture of sulphurlc ac1d from to Reactions 1 and 2. As the table indicates, about 100 sulphur, air and water by the pulp mill may be integrated parts (by weight) of sodium chlorate are reacted with with the basic process so as to permit the pulp mill to from about 55 to about 275 parts of sodium chloride (by I, manufacture bleached pulp starting only with cellulosic weight) and from about 92 to about 276 parts of sulfuric material and the three naturally occurrlng chemical maacid (by weight), and the weight ratio of sodium chloterials, sodium chloride, sulphur and limestone (or lime). ride/ sulfuric acid used in said reaction being from about Many variations of the hereinbefore described proc- 0.6 to about 1 and decreasing as the amount of sodium ess to those skilled in the art are possible without dechlorate used in Reaction 1 increases. The table further parting from the overall inventive concept set out herein, indicates that, in said reaction, the weight ratios of except as defined in the appended claims. sodium chloride to sodium chlorate and sulfuric acid to For example, the generator, with or without an elecsodium chlorate are from about 0.55 to about 2.75 and trolytic cell to produce the feed solution of sodium chlofrom about 0.92 to about 2.76, respectively. rate and sodium chloride, may be used to produce only TABLE I [For 100 lbs. NaClO Lbs. NaClOs, Reaction 1 0 10 20 so Lbs. NaCIOQ, Reaction2 100 90 so 70 60 50 40 30 20 10 0 Lb. 0102 produced 0 6. 3 12. 7 19 25. 4 31. 7 3s 44. 4 50. 7 57.1 63. 4 Lb. C12 produced 200 183 167 143 117 100 83.3 66.7 50 33.3 Lb. NaCl requiretL 275 253 231 209 187 143 121 99 77 55 Lb. HzSO4required 275 258 239 221 203 166 143 130 111 92 Lb. Natsoi produced. 400 373 347 320 293 267 240 213 187 160 133 LbJlzO produced 50. 7 47.3 44 40.6 37. 3 33.9 30.6 27. 2 23.9 20. 5 17. 2

It will be seen from the foregoing that in locations enough chlorine dioxide for the later stages of bleaching where no chemical manufacturing industry exists, the and part of the chlorine for the chlorination stage, the basic process disclosed herein will provide all the chemremaining chlorine required being purchased or manuicals (chlorine, chlorine dioxide, sodium sulphate and factured in a separate electrolytic cell. Alternatively, if sodium hydroxide) necessary for the manufacture of 45 the generator is operated to produce sufiicient chlorine bleached pulp from chemicals (salt, sulphur and limedioxide for the later stages of bleaching and, in addition, stone) which exist as such in nature. a mixture of chlorine and chlorine dioxide which supplies In accordance with this invention there is provided a only a part of the oxidizing power for the chlorination process for producing bleached pulp from cellulosic mastage, additional chlorine and chlorine dioxide may be obterial which comprises the following steps: producing 50 tained from another source. However, it will be apprecisodium sulphate, chlorine and chlorine dioxide from ated that it is preferred to obtain all the oxidizing power sulphuric acid, sodium chloride and sodium chlorate; required for bleaching from the generator. producing an alkaline liquor containing sodium carbonate As another example, if the generator is operated to proand sodium sulphide; using the alkaline liquor so produce less than enough sodium sulphate to produce all the duced in the production of aqueous sodium hydroxide 55 white liquor required for caustic extraction, additional and the provision of aqueous sodium sulphide; using at sodium sulphate, white liquor or caustic soda may be least a part of the aqueous sodium sulphide in the cookobtained from some other source. However, it is preferred ing of cellulosic material to produce a pulp; using the to obtain the total requirement of sodium sulphate from generate-d chlorine and chlorine dioxide in the multi-stage the generator. bleaching of the pulp; using at least a part of the aqueous 60 An important advantage of the processes disclosed sodium hydroxide for extracting partially bleached pulp herein is the extreme flexibility with which they may be from an earlier stage; and using the sodium sulphate so applied. They may be used to supply all or only part of produced together with the used liquor from the cooking the requirements for chlorine, sodium hydroxide and of the cellulosic material for producing the aforemensodium sulphate. Furthermore, they may be practised in tioned alkal ne liquor. 65 such a manner as to utilize only sodium chloride and sul- By combimng all of these steps 1n the manner shown 1n phur as the starting materials for manufacturing chlorine, the accompany ng drawing, which is a flow sheet illustratchlorine dioxide and sodium sulphate, and from the latter mg one embod1ment of the process, the manufacture of hit liquor t i i di h id sodium 1. bleached P p staftlng p y Sodlum at sodium hide and sodium carbonate, or they may use only sodium chloride, sulphuric ac1d and limestone is accomplished. 70 hl id d sulphuric acid as the starting materials or, As an extension of the basic process, the solution of alternatively, only sulphur (or sulphuric acid), sodium sodium chloride and sodium chlorate which is necessary chlorate and sodium chloride. 5 for the manufacture of chlorine dioxide, chlorine and Therefore, these processes may be used with various sodium sulphate may be manufactured in an electrolytic raw materials to make varying amounts of chemicals in cell by the pulp mill. This step, when integrated with the 75 such proportions over a very wide range as are required to result in a minimum total cost for the manufacture of bleached kraft pulp in any particular location with any suitable type of cellulosic raw material, such as Wood, straw, grasses, bamboo, bagasse, etc.

Referring to the drawing, there is schematically shown a sulphuric acid producing plant 10, a sodium chlorate producing plant 11, a generator 12 for producing chlorine, chlorine dioxide and sodium sulphate, a crystallizer 13, an evaporator 14, a chlorine dioxide absorption tower 15, another absorption tower 16, a conventional pulp mill chemical recovery system 17 which includes evaporators, a furnace, a smelt dissolving tank, a recausticizing stage, clarifiers, filters and a kiln, digesters 18, a first or chlorination stage bleach tower 19, caustic extraction towers 20 and 21, later stage bleach towers 22 and 23 and washers 24a-24 these various components being connected together as shown.

One method of operating the basic process is to introduce sodium chlorate into the generator 12 via a line 25 at a rate suificient to produce all the available chlorine required by the pulp bleach plant, along with the required amount of sodium chloride, also added via line 25, to produce the desired ratio of chlorine dioxide to chlorine. Sulphuric acid is introduced into the generator 12 via a line 26 at a rate which will maintain the acidity of the solution in the generator 12 at a suitable concentration, preferably, but not necessarily, at approximately ten normal sulphuric acid. Sufiicient air is introduced into the generator 12 via a line 27 to keep the concentration of chlorine dioxide in the effluent gas below about by volume. The gas leaving the generator 12 via a line 28 is introduced into the absorption tower which is filled with suitable packing in which the gas rises countercurrent to a stream of water introduced via a line 29 and which dissolves chlorine dioxide preferential to produce a chlorine dioxide solution for use in the later stages 22 and 23 of bleaching. The water introduced in the absorption tower 15 is sufiicient to dissolve only the amount of chlorine dioxide required for the later stages 22 and 23 of bleaching to which it is introduced via lines 41, 41a and 4112, while most of the chlorine and some of the chlorine dioxide leaving the first absorption tower 15 with the air is introduced into the second absorption tower 16 via a line 39 up which it flows countercurrent to a flow of water introduced via a line 30 and which is much larger than the flow of water to the first absorption tower 15. The fiow' of water introduced via line 39 dissolves the chlorine and the chlorine dioxide to make a solution enriched in chlorine for use in the first stage 19 of bleaching and introduced thereto via a line 30. The air leaving the second absorption tower 16 may still contain small amounts of chlorine which may be absorbed in an alkaline solution to make hypochlorite either for addition to the first caustic extraction stage or for an additional hypochlorite stage which may be introduced into the bleaching system or may be permitted to go to waste.

The aqueous efiiuent leaving the generator 12 via a line 31 and containing mainly sodium sulphate and sulphuric acid with small amounts of sodium chlorate and sodium chloride is introduced into the crystallizer 13 where the sodium sulphate is separated from the aqueous effiuent and the mother liquor then is returned to the generator 12 via lines 32, 32a and the evaporator 14 for re-use. The evaporator 14 may not be necessary depending upon the type of system used for crystallization.

The sodium sulphate from the crystallizer 13 is introduced into the pulp mill chemical recovery system 17 via a line 33 preferably into the thick black liquor. After burning this mixture in the furnace in which a reducing atmosphere is maintained to reduce sodium sulphate to sodium sulphide, the smelt so produced and containing principally sodium carbonate and sodium sulphide is dissolved in water, clarified, causticized, and returned to the digester 18 via a line 34 as White liquor containing sodium hydroxide, sodium sulphide, and unconverted sodium sulphate and sodium carbonate. Part of the white liquor may be taken 01f via lines 35 and 35a and used in the caustic extraction stages 20 and/ or via lines 35 and 35b and used in the caustic stage 21. After the white liquor has performed its function of dissolving chlorinated lignins and part of the hemicelluloses, the liquor is separated from the pulp in a suitable manner, and the liquor is introduced via lines 36 and 37 into the black liquor line 38 leading to the evaporators of the pulp mill chemical recovery system 17 to be reconverted into white li uor.

The unbleached pulp from digesters 18 is conducted via lines 42 through the washers 24a and line 42a to the first stage bleach tower 19. Black liquor separated from the unbleached pulp in washers 24a is conducted via the line 38 to the evaporators in the pulp mill chemical recovery system 17. The pulp is conducted through the vari\ ous stages of the bleaching system via washers 24b, 24c, 24d, 24c and 24f and lines 43a, 43b, 43c, and 43d and emerges as bleached pulp at 44.

If desirable, the sulphuric acid supplied to the generator 12 via line 26 may be manufactured in the sulphuric acid producing plant 10 from sulphur, air and water, and the sodium chlorate introduced into the generator 12 may be manufactured in the sodium chlorate producing plant 11 by electrolysis of sodium chloride.

Another variation which is possible is to scrub the air leaving the second absorber 16, and which may contain significant amounts of chlorine, with a slurry of lime to produce calcium hypochlorite, which may be used in a sixth hypochlorite bleaching stage, usually inserted between the stages 21 and 22 shown in the drawing. This variation has the advantage that the tower 16 for dissolving the chlorine and chlorine dioxide in water may be smaller, since essentially all the chlorine does not have to be absorbed from the dilute mixture in air.

Another possible variation is to avoid evaporation of Water from the generator-crystallizer system by absorbing S0 in concentrated sulphuric acid in the sulphuric acid manufacturing plant 10, and adding sulphuric acid containing an excess of sulphur trioxide to the generator 12 or to the dilute acid solution coming from the crystallizing system via the line 32. This may diminish the need for evaporation of water from the system and its cost.

It should be noted that it is not necessary to fractionate the chlorine and chlorine dioxide mixture into a chlorinerich fraction for use in an earlier bleaching stage and a chlorine dioxide-rich fraction for use in later bleaching stages as hereinbefore disclosed, although it is preferred to do so. It also should be noted that as few as two bleaching stages separated by a singly caustic extraction stage may be employed, if desired, or, alternatively, more bleaching and caustic extraction stages than are shown in the appended drawing may be used.

Attention also is directed to the .fact that the white liquor from chemical recovery system 17 may be fractionated into a sodium hydroxide-rich fraction for use in the caustic extraction stages and a sodium sulphide-rich fraction for use in the cooking of the cellulosic material. This may be achieved either by actually fractionating the white liquor produced in a conventional pulp mill chemical recovery system, or by treating the smelt solution in such a manner that two separate solutions, one rich in sodium sulphide, and one rich in sodium hydroxide, are produced, or, alternatively, in such a manner that two separate solutions, one containing both sodium hydroxide and sodium sulphide in relatively large amounts, and one comprising mainly sodium hydroxide, are produced. In other words, different types of pulp mill chemical recovery systems than the cnventional type now in use and hereinbefore described may be used without departing from this invention.

,While the production of chlorine, chlorine dioxide and sodium sulphate has been shown as taking place in a system composed of a generator 12 interconnected with a crystallizer 13 and evaporator 14, this is illustrative only, and it is to be understood that other methods and apparatus means for producing these materials may be employed without departing from the scope of this invention.

In the production of chlorine and chlorine dioxide by reaction with sulphuric acid in the presence of chloride ion, the sulphate ion removed from the C10 generation process by the methods referred to above will precipitate as Na SO NaI-ISO sodium sequisulfate, Na SO 10-H 0, etc., or as mixtures of one or more of these sales, any one of which can be used in the pulp mill chemical recovery system 17. The process of my invention includes ClO generation systems which produce and use any of these salts, and in the above description of my invention I have used Na SO and sodium sulphate to include not only that salt itself, but also other sodium sulphate type salts having the S anion in its composition which is produced in the C generation system.

It will be understood that the process hereinbefore described in detail in connection with the appended drawing is merely illustrative of a process embodying this invention and is not to be construed as limiting the invention other than in the appended claims.

What is claimed is:

1. A process for the production of chemicals for the pulping and bleaching of cellulosic materials comprising reacting sodium chlorate, sodium chloride and sulfuric acid to produce chlorine, chlorine dioxide and a sodium sulfate salt, separating the chlorine and chlorine dioxide from said salt, bleaching digested cellulosic pulp with said chlorine and chlorine dioxide, adding the sodium sulfate salt to kraft black liquor produced in the digestion of said cellulosic pump, heating and smelting said kraft black liquor to produce sodium carbonate and sodium sulfide, dissolving the produced sodium carbonate and sodium sulfide in an aqueous solution, reacting the dissolved sodium carbonate and sodium sulfide with lime to produce white liquor, separating the white liquor into separate portions, extracting partially bleached pulp with one portion of said white liquor and returning another portion of said white liquor to a kraft digestion process and digesting cellulosic pulp to produce digested cellulosic pulp and black liquor for the aforementioned heating and smelting step.

2. A process according to claim 1 wherein concentration of the said sulphuric acid in the said production of chlorine dioxide is about ten normal.

3. The process of claim 1 wherein the cellulosic materials are wood chips.

4. The process of claim 1 including the step of producing sodium chlorate by the electrolysis of sodium chloride.

5. The process of claim 1 wherein the white liquor used in extracting the partially bleached pulp is recovered and added to theblack liquor.

6. The process of claim 1, wherein the chlorine and chlorine dioxide are passed through an absorption tower countercurrently to water, thereby producing a first bleaching solution, and the gases which are not dissolved in said absorption tower are passed through another absorption tower, thereby producing a second bleaching solution, said second bleaching solution being used in the first bleaching stage, said first bleaching solution being used in a subsequent bleaching stage.

7. The process of claim 1 wherein:

(a) About parts (by weight) of sodium chlorate are reacted with from about 55 to about 275 parts (by weight) of sodium chloride and with from about 92 to about 276 parts (by weight) of sulfuric acid;

(b) From about 0.6 parts to about 1 part (by weight) of sodium chloride are used for every part of sulfuric acid used in said reaction;

(c) the weight ratios of sodium chloride to sodium chlorate and sulfuric acid to sodium chlorate are from about 0.55 to about 2.75 and from about 0.92 to about 2.76 respectively.

8. The process of claim 1 wherein the produced chlorine and chlorine dioxide is reacted with the digested cellulosic pulp to bleach it in a multi-stage bleaching process.

9. The process of claim 8, wherein said chlorine and chlorine dioxide which have been separated from said salt are passed through an absorption tower countercurrently to water, thereby producing a first bleaching solution, and the gases which are not dissolved in said absorption tower are passed through another absorption tower, thereby producing a second bleaching solution, said second bleaching solution being used in the first bleaching stage, said first bleaching solution being used in a subsequent bleaching stage.

10. In a continuous cyclic process for producing bleached wood pulp wherein the black liquor from the digested unbleached wood pulp by the kraft process is converted to white liquor in a pulp mill chemical smelt recovery system and used to digest wood chips, and the digested unbleached wood pulp is then passed through multi-stage sequential bleaching and extraction operations the improvement which comprises, extracting the bleached pulp in the extraction operation with at least part of the white liquor and returning the spent extraction liquor to the said pulp mill chemical recovery system for conversion to white liquor.

References Cited UNITED STATES PATENTS 2,129,719 9/1938 Vincent 162-87 2,364,177 12/1944 Wells l6289 X 2,587,064 2/1952 Rapson 162-88 X 3,020,197 2/1962 Schuber l62-88 X 3,052,592 9/1962 Eberhardt l6287 X OTHER REFERENCES Trivedi et al.: The Paper Industry and Paper World, January, 1948, Extraction Treatments in Bleaching Aspen Neutral Sulphite Semichemical Pulp, pages 1443- 1453 (pages 1449 and 1450 relied upon).

S. LEON BASHORE, Primary Examiner.

DONALL H. SYLVESTER, Examiner. 

1. A PROCESS FOR THE PRODUCTION OF CHEMICALS FOR THE PULPING AND BLEACHING OF CELLULOSIC MATERIALS COMPRISING REACTING SODIUM CHLORATE, SODIUM CHLORIDE AND SULFURIC ACID TO PRODUCE CHLORINE, CHLORINE DIOXIDE AND A SODIUM SULFATE SALT, SEPARATING THE CHLORINE AND CHLORINE DIOXIDE FROM SAID SALT, BLEACHING DISGESTED CELLULOSIC PLUP WITH SAID CHLORINE AND CHLORINE DIOXIDE, ADDDING THE SODIUM SULFATE SALT TO KRAFT BLACK LIQUOR PRODUCED IN THE DIGESTION OF SAID CELLULOSIC PUMP, HEATING AND SMELTING SAID KRAFT BLACK LIQUOR TO PRODUCE SODIUM CARBONATE AND SODIUM SULFIDE, DISSOLVING THE PRODUCED SODIUM CARBONATE AND SODIUM SULFIDE IN AN AQUEOUS SOLUTION, REACTING THE DISSOLVED SODIUM CARBONATE AND SODIUM SULFIDE WITH LIME TO PRODUCE WHITE LIQUOR, SEPARATING THE WHITE LIQUOR INTO SEPARATE PORTIONS, EXTRACTING PARTIALLY BLEACHED PULP WITH ONE PORTION OF SAID WHITE LIQUOR AND RETURNING ANOTHER PORTION OF SAID WHITE LIQUOR TO A KRAFT DIGESTION PROCESS AND DIGESTING CELLULOSIC PULP TO PRODUCE DIGESTED CELLULOSIC PULP AND BLACK LIQUOR FOR THE AFOREMENTIONED HEATING AND SMELTING STEP. 